Ecological risk thresholds for Zn in Chinese soils

The environmental risk threshold of a pollutant is a yardstick to measure soil environmental quality. The derivation of ecological risk thresholds of the heavy metal zinc (Zn) in soil environments based on up-to-date ecological risk assessments plays an important role in soil protection policy. According to regional soil classification, different representative soils with various degrees of acidity and alkalinity were selected, and a data set comprising ecotoxicities of Zn to 21 different test endpoints (plants, soil fauna, microorganisms, etc.) found in representative farmland soils of China was compiled based on new and published data to determine toxicological limits of Zn effects on endpoints. These limits were derived from fitted dose-response model parameters and indicated by EC₁₀ values (the effective concentrations of Zn that inhibit 10% of endpoint bioactivity and also represents the toxicity threshold of Zn in this study) ranging from 36 mg·kg^-1 to 682 mg·kg^-1. The hormesis effect appeared in the dose-response curve of Zn, for example, the relative Chinese cabbage growth reached more than 120% at most. Zn concentrations added in toxicity tests were also corrected for aging and leaching effects in order to more accurately reflect field conditions. The hazardous concentrations for 5% of the species affected (HC₅) were derived by the species sensitivity distribution (SSD) approach for four major types of Chinese soils: acidic (38 mg·kg^-1), neutral (106 mg·kg^-1), alkaline (217 mg·kg^-1), and alkaline calcareous soils (155 mg·kg^-1). Prediction models of ecological risk thresholds for Zn based on soil properties were generated, such as logHC₅ = 0.564 + 0.218pH + 0.097OC (R² = 0.790,p < 0.001). The predicted models based on lab test data were verified in the field, and the measured field data fell within two-fold of the prediction intervals. This work provides a scientific framework for developing soil-specific guidance on Zn toxicity thresholds.